stock and water systems of the paper machine

Ulrich Weise, Jukka Terho and Hannu PaulapuroChapter 5

Stock and water systems of the papermachine5.1 DefinitionsThe following terms are commonly used to specify certain areas and systems as part of the entirepaper mill water system:

Short circulation: The system in which paper machine wire water is separated from the stockin web forming and used for dilution of the thick stock to be delivered to the headbox.

Long circulation: The system in which excess white water from the short circulation and otherwaters are collected at the paper machine (PM) and used for stock dilution and other purposes instock preparation. Within the long circulation loop, usually fiber recovery and water cleaningequipment is installed.

Approach flow system: The system extends from the machine chest to the headbox lip. Themain purpose is to meter and dilute the stock including blending with other components likefillers, chemicals, and additives unless not already added in stock preparation. Then, thelow-consistency stock is pumped and screened before feeding to the headbox. Stock cleaning byhydrocyclones and deaeration can be included.

Stock preparation: Stock preparation or "stock prep" includes mechanical treatment of thestock before the machine chest, proportioning, and blending of the main stock components.Stock preparation begins with repulping or the dilution of pulp from integrated mill operations atthe pulp storage towers and ends at the machine chest.

5.2 Design principles5.2.1 Elements and operationsThe purpose of the stock and water systems is to supply the paper machine (PM) with stock andwater in such way that

- The quantity of supplied stock is sufficient for the production capacity of the PM- The supply is even and of such quality in order to reach a high PM productivity- The product at the reel meets the given quality parameters.

Additionally, process design and operations are carried out in such a way that theabove-mentioned aims are met at minimum cost. Hence, the process design has to be optimizedaccording to energy consumption and stock quality refinement in particular. Other constraintshave to be considered like process water quality, especially at low fresh water consumption, orpulp quality characteristics. Finally, these aims have to be met at all possible operationconditions. The process and the PM are usually designed so that their highest efficiencies arereached with the most produced grade, which is preferably in the middle of the range of gradesaimed for. A small increase in running time efficiency on the main grade is often more beneficialthan aiming for highest efficiency with all possible grades. The target is to achieve the maximumtotal production efficiency. This requires a good knowledge of the product mix to be realizedbefore building or rebuilding the mill. Thus, process and installed equipment can be tailored forthe specific grade, which gives advantage in specific consumption of energy and other resources.

Ulrich Weise, Jukka Terho and Hannu PaulapuroChapter 5

Stock and water systems of the papermachine5.1 DefinitionsThe following terms are commonly used to specify certain areas and systems as part of the entirepaper mill water system:

Short circulation: The system in which paper machine wire water is separated from the stockin web forming and used for dilution of the thick stock to be delivered to the headbox.

Long circulation: The system in which excess white water from the short circulation and otherwaters are collected at the paper machine (PM) and used for stock dilution and other purposes instock preparation. Within the long circulation loop, usually fiber recovery and water cleaningequipment is installed.

Approach flow system: The system extends from the machine chest to the headbox lip. Themain purpose is to meter and dilute the stock including blending with other components likefillers, chemicals, and additives unless not already added in stock preparation. Then, thelow-consistency stock is pumped and screened before feeding to the headbox. Stock cleaning byhydrocyclones and deaeration can be included.

Stock preparation: Stock preparation or "stock prep" includes mechanical treatment of thestock before the machine chest, proportioning, and blending of the main stock components.Stock preparation begins with repulping or the dilution of pulp from integrated mill operations atthe pulp storage towers and ends at the machine chest.

5.2 Design principles5.2.1 Elements and operationsThe purpose of the stock and water systems is to supply the paper machine (PM) with stock andwater in such way that

- The quantity of supplied stock is sufficient for the production capacity of the PM- The supply is even and of such quality in order to reach a high PM productivity- The product at the reel meets the given quality parameters.

Additionally, process design and operations are carried out in such a way that theabove-mentioned aims are met at minimum cost. Hence, the process design has to be optimizedaccording to energy consumption and stock quality refinement in particular. Other constraintshave to be considered like process water quality, especially at low fresh water consumption, orpulp quality characteristics. Finally, these aims have to be met at all possible operationconditions. The process and the PM are usually designed so that their highest efficiencies arereached with the most produced grade, which is preferably in the middle of the range of gradesaimed for. A small increase in running time efficiency on the main grade is often more beneficialthan aiming for highest efficiency with all possible grades. The target is to achieve the maximumtotal production efficiency. This requires a good knowledge of the product mix to be realizedbefore building or rebuilding the mill. Thus, process and installed equipment can be tailored forthe specific grade, which gives advantage in specific consumption of energy and other resources.

Chapter 5 Stock and water systems of the paper machine

Papermaking Part 1, Stock Preparation and Wet End - Page 1

On the other hand, producing grades other than those initially considered is less economical oreven impossible. Generally speaking, the larger the PM production is, the more narrow is thewindow of grades which can be produced on this machine. Hence, large and fast runningmachines are usually "single-grade machines."

In order to avoid unfavorable interferences and in order to gain steady operation of certainequipment or sub-processes, these units are, at least to some degree, de-coupled from eachother. This is usually achieved by buffers with sufficient capacity or by the possibility to switch orto connect process streams differently. The higher the amount of storage or buffer capacity is, thelarger the variation may be, which can be leveled out. However, the smoothening effect of largebuffers on very low frequency variation can be limited. On a multigrade machine, the time neededfor changing grades should be short and the amount of broke produced during the grade changeshould be small. Hence, stock chests should be relatively small and recirculations short. The totalproduction efficiency on a multigrade machine is usually lower than for especially designedsingle-grade processes. Table 1 shows typical total efficiencies for modern paper mills; theefficiency figure considers scheduled and unscheduled downtime and broke.

Table 1. Typical total efficiencies of modern paper machines.

Grade produced Total efficiencyNewsprintSupercalendered paperLightweight coated paperWoodfree paperTissue paperLinerboardFolding boxboard

90%90%82%92%85%85%82%

The sub-processes of the stock and water systems of the paper machine are:- Stock preparation- Stock approach flow system- White water and fiber recovery system- Broke system.

Figures 1 and 2 show stock preparation and short circulation as blackbox models. Forprocess design as well as for troubleshooting in mill practice, it is useful to be aware of theborders and interfaces between the sub-processes and operations, especially in complex andsophisticated systems. Addressing the criteria, needs, and concerns for each process stepmakes it evident as to whether all pieces are suitable to meet their particular goal. Designing theprocesses accordingly will maximize profitability. Categorizing the activities and defining theprocess steps is also known as system analysis. Possible bottlenecks in processes can beidentified. In an interconnected system, the entire system is only as good as its weakest part. Inthis respect, malfunction or disturbances originating from auxiliary equipment, fittings, piping,controls, etc., can spoil process performance significantly, even if the main equipment is wellchosen and, as such, operating perfectly. Separation of processes into functional units makescomplex and sophisticated systems more comprehensible and easier to understand, whichmeans a potential improvement in controllability, system stability, operation safety, etc. Clearlystructured processes also ease troubleshooting because reason and cause of problems can bemore quickly and easily separated and identified.

Figure 1. Stock preparation, system principle.

On the other hand, producing grades other than those initially considered is less economical oreven impossible. Generally speaking, the larger the PM production is, the more narrow is thewindow of grades which can be produced on this machine. Hence, large and fast runningmachines are usually "single-grade machines."

In order to avoid unfavorable interferences and in order to gain steady operation of certainequipment or sub-processes, these units are, at least to some degree, de-coupled from eachother. This is usually achieved by buffers with sufficient capacity or by the possibility to switch orto connect process streams differently. The higher the amount of storage or buffer capacity is, thelarger the variation may be, which can be leveled out. However, the smoothening effect of largebuffers on very low frequency variation can be limited. On a multigrade machine, the time neededfor changing grades should be short and the amount of broke produced during the grade changeshould be small. Hence, stock chests should be relatively small and recirculations short. The totalproduction efficiency on a multigrade machine is usually lower than for especially designedsingle-grade processes. Table 1 shows typical total efficiencies for modern paper mills; theefficiency figure considers scheduled and unscheduled downtime and broke.

Table 1. Typical total efficiencies of modern paper machines.

Grade produced Total efficiencyNewsprintSupercalendered paperLightweight coated paperWoodfree paperTissue paperLinerboardFolding boxboard

90%90%82%92%85%85%82%

The sub-processes of the stock and water systems of the paper machine are:- Stock preparation- Stock approach flow system- White water and fiber recovery system- Broke system.

Figures 1 and 2 show stock preparation and short circulation as blackbox models. Forprocess design as well as for troubleshooting in mill practice, it is useful to be aware of theborders and interfaces between the sub-processes and operations, especially in complex andsophisticated systems. Addressing the criteria, needs, and concerns for each process stepmakes it evident as to whether all pieces are suitable to meet their particular goal. Designing theprocesses accordingly will maximize profitability. Categorizing the activities and defining theprocess steps is also known as system analysis. Possible bottlenecks in processes can beidentified. In an interconnected system, the entire system is only as good as its weakest part. Inthis respect, malfunction or disturbances originating from auxiliary equipment, fittings, piping,controls, etc., can spoil process performance significantly, even if the main equipment is wellchosen and, as such, operating perfectly. Separation of processes into functional units makescomplex and sophisticated systems more comprehensible and easier to understand, whichmeans a potential improvement in controllability, system stability, operation safety, etc. Clearlystructured processes also ease troubleshooting because reason and cause of problems can bemore quickly and easily separated and identified.

Figure 1. Stock preparation, system principle.



The number of used pulp components depends on their availability and on the productproperties desired. Accordingly, in stock preparation, the fiber furnish is determined by:

- Selection and proportion of the stock components- Improvement and development of the fibers, i.e., beating.

Chapters 3 and 4 describe in detail the main operations like slushing, defibration, and refiningof pulp. The consumption of fresh water in stock preparation is very low if not zero. Fresh water isused for the dilution of chemicals and eventually as supplement water in startup situations. In asimple case, stock preparation consists of dilution of a single pulp component and mixing it withrecovered fiber and broke.

Figure 2 shows a typical short circulation as a blackbox model; the streams of air fromdeaeration and filler are optional according to the paper produced. The "rejects" stream here alsoincludes the end-stage screen accept, if this is fed into the couch pit. The heating energy addedto the wire pit controls the process temperature; its application can be restricted to the startupsituation. All fluid streams vary purposely and arbitrarily in flow, concentration, the composition ofsuspended and dissolved materials, and temperature to some extent. In this respect, thecomponent with the most impact on this system is the thick stock flow.

Figure 2. Short circulation, system principle.

The main operations in the approach flow system are:- Dilution to headbox consistency- Removal of product and production disturbing contaminants (solids and air)- Conditioning with chemicals and additives- Feeding the headbox- Supply of additional water for PM cross-profile control in case of a headbox dilution system.

Figure 3 shows the approach flow system of a printing paper mill. The thick stock is pumpedfrom the machine chest (1) for dilution at the wire pit (2). After the thick stock pump a shut-offvalve, a magnetic flowmeter, and a connector for the consistency measurement are shown. Thus,the basis weight is here controlled by the speed of the thick stock pump. At the stock mixing zoneof the wire pit (2), right before the stock fan pump (3), the additive dosage points are shown. Thestock fan pump (3) feeds the first stage cleaner banks (4), from where the accept flows to thedeaeration tank (5). The air from the stock deaeration tank (5) and the headbox dilution waterdeaeration tank (not shown) passes through an on/off valve and the condenser to the first-stagevacuum pump (6) followed by a water separator. On the small branch pointing to the right side,the vacuum breaker valve is shown. The second stage vacuum pump followed by a waterseparator is partly hidden. The stock flows from the deaeration tank (5) to the headbox fan pump(7). There are connections shown for filler and additive dosage right before the fan pump (7).There are two horizontally mounted pressure screens (8), which are equipped each with anautomatic shut-off valve on the feed side and a manual shut-off valve at the accept side. Rejectcollection and treatment are not shown here. From the screens (8), the machine stock flows tothe headbox of the PM. Retention aid is dosed via a ring pipe header and several radial dosagepoints (9).

Figure 3. Stock approach flow system of a fine paper machine (refer to text for itemization ofposition numbers).

5.2.2 System stability

The number of used pulp components depends on their availability and on the productproperties desired. Accordingly, in stock preparation, the fiber furnish is determined by:

- Selection and proportion of the stock components- Improvement and development of the fibers, i.e., beating.

Chapters 3 and 4 describe in detail the main operations like slushing, defibration, and refiningof pulp. The consumption of fresh water in stock preparation is very low if not zero. Fresh water isused for the dilution of chemicals and eventually as supplement water in startup situations. In asimple case, stock preparation consists of dilution of a single pulp component and mixing it withrecovered fiber and broke.

Figure 2 shows a typical short circulation as a blackbox model; the streams of air fromdeaeration and filler are optional according to the paper produced. The "rejects" stream here alsoincludes the end-stage screen accept, if this is fed into the couch pit. The heating energy addedto the wire pit controls the process temperature; its application can be restricted to the startupsituation. All fluid streams vary purposely and arbitrarily in flow, concentration, the composition ofsuspended and dissolved materials, and temperature to some extent. In this respect, thecomponent with the most impact on this system is the thick stock flow.

Figure 2. Short circulation, system principle.

The main operations in the approach flow system are:- Dilution to headbox consistency- Removal of product and production disturbing contaminants (solids and air)- Conditioning with chemicals and additives- Feeding the headbox- Supply of additional water for PM cross-profile control in case of a headbox dilution system.

Figure 3 shows the approach flow system of a printing paper mill. The thick stock is pumpedfrom the machine chest (1) for dilution at the wire pit (2). After the thick stock pump a shut-offvalve, a magnetic flowmeter, and a connector for the consistency measurement are shown. Thus,the basis weight is here controlled by the speed of the thick stock pump. At the stock mixing zoneof the wire pit (2), right before the stock fan pump (3), the additive dosage points are shown. Thestock fan pump (3) feeds the first stage cleaner banks (4), from where the accept flows to thedeaeration tank (5). The air from the stock deaeration tank (5) and the headbox dilution waterdeaeration tank (not shown) passes through an on/off valve and the condenser to the first-stagevacuum pump (6) followed by a water separator. On the small branch pointing to the right side,the vacuum breaker valve is shown. The second stage vacuum pump followed by a waterseparator is partly hidden. The stock flows from the deaeration tank (5) to the headbox fan pump(7). There are connections shown for filler and additive dosage right before the fan pump (7).There are two horizontally mounted pressure screens (8), which are equipped each with anautomatic shut-off valve on the feed side and a manual shut-off valve at the accept side. Rejectcollection and treatment are not shown here. From the screens (8), the machine stock flows tothe headbox of the PM. Retention aid is dosed via a ring pipe header and several radial dosagepoints (9).

Figure 3. Stock approach flow system of a fine paper machine (refer to text for itemization ofposition numbers).

5.2.2 System stability



Production conditions and parameters can be quite different according to the wide range ofdifferent paper and board grades. Papermaking succeeds even with highly loaded process waterin closed cycle, i.e., effluent-free, production. However, process conditions have to be stable toreach the required product quality and a high production efficiency. Steadiness and uniformity offlow are vital and are determined by the design as well as by the operation of the process.Especially in modern operations with high-speed machines and with low fresh waterconsumption, the physical system stability is as important as the chemical stability of the watersystems and the wet end. Attention is paid to the variation of several parameters, separatelyacting or interacting with each other, when designing or improving the operation of the flowapproach system and the wet end. Problems can be caused by variation of:

- Flow speed- Pressure (pulsation)- Temperature- Flow conditions and the degree of turbulence- Consistency- Furnish composition and its homogeneity including inorganic solids- Specific surface of fibers, content of fines, freeness- Charge content, cationic demand, pH- Content and distribution of chemicals and additives- Content and distribution of undesired substances and contaminants including air.

In the stock and water system of the PM, the importance of steadiness and uniformity of flowincreases when approaching the sheet forming. Unevenness in any flow can occur for each ofthe above-mentioned parameters:

- In the cross-direction of the flow- In the flow direction.

The latter means a time variation of the flow, which appears in:- Short term: in frequencies > 1 Hz, or in wavelengths < 1 s- Long term: in frequencies < 1 Hz, or in wavelengths > 1 s.

Additionally to these periodical (deterministic) variations, there are random (stochastic)variations in the short term as well as in long term.5.2.2.1 Grade changesContrary to the principle of steady operation are grade changes on the PM. Carrying out gradechanges has to be considered carefully when designing the approach flow system and the longcirculation of a paper or board machine to ensure stable operation over the entire product range.This applies for so-called "single-grade" as well as for "multigrade" machines; on the single-grademachine, the difference between the produced grades is small. The following points are importanton multigrade machines:

- Quick response upon changes to gain a short transition period when changing to anothergrade. This means an improvement in production time efficiency and less broke. Large storagecapacity dampens the response.

- Adaptation without production disturbance. Machine runnability or production should not belower right after the grade change compared to steady-state operation.

- Fast leveling-out of variations, e.g., caused by instability in the wet end chemistry. Besidesthe PM runnability, product quality also should not be on a lower level after the grade change

Production conditions and parameters can be quite different according to the wide range ofdifferent paper and board grades. Papermaking succeeds even with highly loaded process waterin closed cycle, i.e., effluent-free, production. However, process conditions have to be stable toreach the required product quality and a high production efficiency. Steadiness and uniformity offlow are vital and are determined by the design as well as by the operation of the process.Especially in modern operations with high-speed machines and with low fresh waterconsumption, the physical system stability is as important as the chemical stability of the watersystems and the wet end. Attention is paid to the variation of several parameters, separatelyacting or interacting with each other, when designing or improving the operation of the flowapproach system and the wet end. Problems can be caused by variation of:

- Flow speed- Pressure (pulsation)- Temperature- Flow conditions and the degree of turbulence- Consistency- Furnish composition and its homogeneity including inorganic solids- Specific surface of fibers, content of fines, freeness- Charge content, cationic demand, pH- Content and distribution of chemicals and additives- Content and distribution of undesired substances and contaminants including air.

In the stock and water system of the PM, the importance of steadiness and uniformity of flowincreases when approaching the sheet forming. Unevenness in any flow can occur for each ofthe above-mentioned parameters:

- In the cross-direction of the flow- In the flow direction.

The latter means a time variation of the flow, which appears in:- Short term: in frequencies > 1 Hz, or in wavelengths < 1 s- Long term: in frequencies < 1 Hz, or in wavelengths > 1 s.

Additionally to these periodical (deterministic) variations, there are random (stochastic)variations in the short term as well as in long term.5.2.2.1 Grade changesContrary to the principle of steady operation are grade changes on the PM. Carrying out gradechanges has to be considered carefully when designing the approach flow system and the longcirculation of a paper or board machine to ensure stable operation over the entire product range.This applies for so-called "single-grade" as well as for "multigrade" machines; on the single-grademachine, the difference between the produced grades is small. The following points are importanton multigrade machines:

- Quick response upon changes to gain a short transition period when changing to anothergrade. This means an improvement in production time efficiency and less broke. Large storagecapacity dampens the response.

- Adaptation without production disturbance. Machine runnability or production should not belower right after the grade change compared to steady-state operation.

- Fast leveling-out of variations, e.g., caused by instability in the wet end chemistry. Besidesthe PM runnability, product quality also should not be on a lower level after the grade change



compared to steady-state operation.- Stable operation of the system and all its components at all operation levels. Flows,

concentrations, and chemistry can differ significantly between different grades, and this has to bepaid attention to in design and operation. Stability is to be achieved hydrodynamically,mechanically, and electrically. Control loops have to be tuned accordingly.

The short circulation shows, according to Norman's simplified short circulation model1, adynamic response for the normalized time t

V=Q2, where V is the wire pit volume and Q2 is the

dilution water flow for the thick stock (Q0, c0) in the wire pit. Figure 4 b shows the dynamicresponse for different retention values R, when starting the system in Fig. 4 a with clear water.Consider that each stock fraction has a different retention value. The curves in Fig. 4 b could be,for example, the retention curves for different Bauer McNett fractions of a certain stockcomposition. Their different response causes variations in the basis weight as well as in the webcomposition as a reaction to quality or quantity changes in the thick stock flow. An increase inthick stock feed leads to a quicker change in the long fiber fraction of the web and to a slowerchange in the fines and filler content. Vice versa, when decreasing the thick stock flow, fines andfillers are over-represented in the web, due to the less retained material circulating1. This cancause oscillation in the product properties, especially after grade changes, which can reduce thetotal efficiency of the PM.

If a change between particular grades is not possible without major material losses orprocess complications, emptying and washing of the system is necessary. In this case, theproduction must be scheduled accordingly. Such washing periods are carried out, for example, incolored paper production, where the dyestuff is changed grade by grade from light to darkshades. After producing the darkest shade, the system is cleaned and production can be startedagain with the lightest shade.

Figure 4. (a.) Short circulation model. (b.) Dynamic response for different retention values R as afunction of normalized time1.

5.2.2.2 Pressure variations and pulsationOf the parameters possibly varying in short-term pressure variations or pulsation are the mostcritical in the short circulation. High-speed, single-layer machines for printing paper productionare affected in particular. Changes in pressure alter the headbox pressure causing undesiredvariations in the headbox lip flow and in the fiber orientation, etc. In the PM wet end, flowvariations are experienced as "barring." Pressure pulses can cause segregation of fibers, fines,and fillers in the suspension, which cause small-scale stock concentration variations and, thus,affect the formation of the sheet. Also fiber spinning, plugging of screens, and deposits can becaused. These variations not only reduce the flow uniformity in the machine-direction, but alsomake the control of the cross-direction profile more difficult. As a result, the runnability, thus theproduction efficiency of the PM is reduced. Besides runnability, also the product quality can bereduced and, in many cases, printability in particular.

Couching several plies like in board machines dampens significantly small-scale variation,typically basis weight variations, which might be critical for a single-layered sheet.

Pulsation analysisThe stock approach system is a collection of elements that can

- create,- transmit,

compared to steady-state operation.- Stable operation of the system and all its components at all operation levels. Flows,

concentrations, and chemistry can differ significantly between different grades, and this has to bepaid attention to in design and operation. Stability is to be achieved hydrodynamically,mechanically, and electrically. Control loops have to be tuned accordingly.

The short circulation shows, according to Norman's simplified short circulation model1, adynamic response for the normalized time t

V=Q2, where V is the wire pit volume and Q2 is the

dilution water flow for the thick stock (Q0, c0) in the wire pit. Figure 4 b shows the dynamicresponse for different retention values R, when starting the system in Fig. 4 a with clear water.Consider that each stock fraction has a different retention value. The curves in Fig. 4 b could be,for example, the retention curves for different Bauer McNett fractions of a certain stockcomposition. Their different response causes variations in the basis weight as well as in the webcomposition as a reaction to quality or quantity changes in the thick stock flow. An increase inthick stock feed leads to a quicker change in the long fiber fraction of the web and to a slowerchange in the fines and filler content. Vice versa, when decreasing the thick stock flow, fines andfillers are over-represented in the web, due to the less retained material circulating1. This cancause oscillation in the product properties, especially after grade changes, which can reduce thetotal efficiency of the PM.

If a change between particular grades is not possible without major material losses orprocess complications, emptying and washing of the system is necessary. In this case, theproduction must be scheduled accordingly. Such washing periods are carried out, for example, incolored paper production, where the dyestuff is changed grade by grade from light to darkshades. After producing the darkest shade, the system is cleaned and production can be startedagain with the lightest shade.

Figure 4. (a.) Short circulation model. (b.) Dynamic response for different retention values R as afunction of normalized time1.

5.2.2.2 Pressure variations and pulsationOf the parameters possibly varying in short-term pressure variations or pulsation are the mostcritical in the short circulation. High-speed, single-layer machines for printing paper productionare affected in particular. Changes in pressure alter the headbox pressure causing undesiredvariations in the headbox lip flow and in the fiber orientation, etc. In the PM wet end, flowvariations are experienced as "barring." Pressure pulses can cause segregation of fibers, fines,and fillers in the suspension, which cause small-scale stock concentration variations and, thus,affect the formation of the sheet. Also fiber spinning, plugging of screens, and deposits can becaused. These variations not only reduce the flow uniformity in the machine-direction, but alsomake the control of the cross-direction profile more difficult. As a result, the runnability, thus theproduction efficiency of the PM is reduced. Besides runnability, also the product quality can bereduced and, in many cases, printability in particular.

Couching several plies like in board machines dampens significantly small-scale variation,typically basis weight variations, which might be critical for a single-layered sheet.

Pulsation analysisThe stock approach system is a collection of elements that can

- create,- transmit,



- dampen or amplify

pressure variations. The effect of these pulses can be studied from the basis weight variations,either on-line or off-line. A more direct measurement of pulsation is possible by installingpressure transducers at pipes or by taking vibration measurements with accelerometers outsideof pipes or machines. The occurring frequencies and amplitudes characterize the pulsation.Usually, the higher the amplitude is, the greater is the disturbance. Signal analysis including theFast Fourier Transformation (FFT) is used to extract frequency components2. Figure 5 shows thetypical frequency bands of sources in which the disturbances occur at the short circulation.

Figure 5. Frequency bands of hydraulic pulsation sources in the short circulation.

The detrimental effect of mechanical vibrations cannot be underestimated. Sufficient supportof pipes and solid foundations of machinery are important3. Separate foundations andsubstructure of the headbox and wire section reduce vibrations in the wet end. In the approachflow system, a well-designed framework and proper mounting of the piping gives enough supportto avoid vibrations but allowing for thermal expansion.

Origin of pressure variationsAll rotating equipment in contact with stock can contribute to pulses, which affect the uniformity ofthe headbox lip flow, e.g.:

- Stock and headbox fan pump- Pressure screen(s)- Headbox rectifier rolls.

If pulses occur at the rotational frequency, usually a mechanical problem is indicated likeout-of-roundness caused by damage or improper installation or misalignment due to wornbearings. Pulses can also occur at the multiples of the rotational frequency. For example, for thepressure screen, the possibly occurring frequencies are according to the number, the geometry,and the interaction of the rotating elements with the screen. The amplitude of pulses depends onthe design, the manufacturing precision, and the degree of wear. Besides mechanical problems,also hydraulic overloading can cause or amplify pulsation. Air in stock has a similar effect.

Stochastic (random) pressure variations can have different origins, including:- Air in stock and air pockets in the system, e.g., at pipe bends- Mixing and dilution sites, e.g., by the headbox re-circulation loop- Controllers and faulty measurements.

5.2.2.3 Stock consistency variationsThe constant thick stock feed flow to the PM has been a major concern since the development ofcontinuous papermaking. Long-term thick stock concentration variations can be caused by:

- Large and sudden variations in the component flows to the blend chest- Problems with consistency measurement and control by:

- Faulty measurements- Insufficiently tuned controllers- Strong pressure variations in the dilutionwater header- Other physical problems such as hysteresis of valves and pressure variations incontrol air, etc.

- Wrong design of the machine chest or insufficient agitation.

Figure 6. Stock Sankey diagram of supercalendered (SC) paper machine at design production.

- dampen or amplify

pressure variations. The effect of these pulses can be studied from the basis weight variations,either on-line or off-line. A more direct measurement of pulsation is possible by installingpressure transducers at pipes or by taking vibration measurements with accelerometers outsideof pipes or machines. The occurring frequencies and amplitudes characterize the pulsation.Usually, the higher the amplitude is, the greater is the disturbance. Signal analysis including theFast Fourier Transformation (FFT) is used to extract frequency components2. Figure 5 shows thetypical frequency bands of sources in which the disturbances occur at the short circulation.

Figure 5. Frequency bands of hydraulic pulsation sources in the short circulation.

The detrimental effect of mechanical vibrations cannot be underestimated. Sufficient supportof pipes and solid foundations of machinery are important3. Separate foundations andsubstructure of the headbox and wire section reduce vibrations in the wet end. In the approachflow system, a well-designed framework and proper mounting of the piping gives enough supportto avoid vibrations but allowing for thermal expansion.

Origin of pressure variationsAll rotating equipment in contact with stock can contribute to pulses, which affect the uniformity ofthe headbox lip flow, e.g.:

- Stock and headbox fan pump- Pressure screen(s)- Headbox rectifier rolls.

If pulses occur at the rotational frequency, usually a mechanical problem is indicated likeout-of-roundness caused by damage or improper installation or misalignment due to wornbearings. Pulses can also occur at the multiples of the rotational frequency. For example, for thepressure screen, the possibly occurring frequencies are according to the number, the geometry,and the interaction of the rotating elements with the screen. The amplitude of pulses depends onthe design, the manufacturing precision, and the degree of wear. Besides mechanical problems,also hydraulic overloading can cause or amplify pulsation. Air in stock has a similar effect.

Stochastic (random) pressure variations can have different origins, including:- Air in stock and air pockets in the system, e.g., at pipe bends- Mixing and dilution sites, e.g., by the headbox re-circulation loop- Controllers and faulty measurements.

5.2.2.3 Stock consistency variationsThe constant thick stock feed flow to the PM has been a major concern since the development ofcontinuous papermaking. Long-term thick stock concentration variations can be caused by:

- Large and sudden variations in the component flows to the blend chest- Problems with consistency measurement and control by:

- Faulty measurements- Insufficiently tuned controllers- Strong pressure variations in the dilutionwater header- Other physical problems such as hysteresis of valves and pressure variations incontrol air, etc.

- Wrong design of the machine chest or insufficient agitation.

Figure 6. Stock Sankey diagram of supercalendered (SC) paper machine at design production.



Figure 7. Water Sankey diagram of supercalendered (SC) paper machine at design production.

Changes in the amount of stock circulating in the short circulation, i.e., by changes in the wireretention, have an effect on the stock concentration in the headbox. This includes slow variationsin retention, which might be initiated by changes in the amount, size distribution, or charge of thefillers and fines fraction, changes in the amount of dissolved and detrimental substances, or incationic charge demand, temperature, pH, etc.5.2.2.4 Headbox approach flow stabilityPipe bends, joints, and installations cause pipe flow disturbances, which appear as turbulenceand vortices causing a head loss4,5. Especially immediately before the headbox, secondary flowsor vortices have to be avoided as far as possible to ensure the most even flow conditions. Thenumber of bends in the pipe between the machine screen(s) and headbox should be as small aspossible, also for lessening the costs. Figure 3 shows one possible solution. The last bend beforethe headbox has a radius of at least three times the pipe diameter. If that is not possible for somereason, the pipe should be carried out as a so-called "hydraulic knee," where the pipe diameter isreduced over the bend. This solution is usually more expensive; however, in any case, cavitationand flow separation must be avoided. The bend is followed by a straight section of about fivetimes the pipe diameter right before the tapered inlet header of the headbox (if an attenuation unitor other special headbox approach is not used)6. No control valves or other measuring devicesare installed in the headbox feed pipe. Properties of the headbox feed stock are measured afterthe headbox at the recirculation line. See also the section about stock transport.

5.2.3 Stock and water balanceDetermining the stock and water balance of the paper mill is the starting point for any newprocess design as well as for an existing system analysis. The balances can vary substantially fordifferent situations, e.g., production of the lowest and the highest basis weight or in the case ofweb breaks. An illustrative manner of representing balances is in the form of a Sankey-diagramas shown in Figs. 6 and 7 for a supercalendered paper line in steady operation. Figure 6 showsthe flow of solids relative to the amount of paper at the reel, and Fig. 7 shows the specific waterflow for the same situation. Note that, for this grade, there is a large amount of solids in the shortcirculation in this case, at a retention of 54%. For clarity reasons, the figure shows the cleanerand screening system as one block each and the PM shower water system is very simplified.Collected waters are of different quality, and they are not fed all together into the white water(WW) tank, as shown in Figs. 6 and 7. Note also that the PM comprises a dilution water headbox.Dilution at the second and third cleaner stages is done with wire water, which is shown by thesmaller of the two parallel streams between the blocks "wire pit and fan pump" and "cleanersystem" in Figs. 6 and 7. The paper mill surplus water is usually fed upstream, like in this case,into the mechanical pulp production unit.

5.2.4 Multi-ply and multilayer systemsThe number of separate stock approach flow systems increases according to the number ofdifferent stock components used either in multi-ply or multilayer paper and board production.Figures 8 through 10 show the block diagrams for a printing paper-producing machine, amultilayer-producing PM like tissue or linerboard, and a multi-ply board or linerboard machine.

The multilayered sheet is produced from one multichannel headbox and one former; thus,there is only one wire pit (see Fig. 9). The multichannel headbox combines two or three stockflows into one jet leaving the headbox lip. This flow consists typically of two different furnishes,which means in the case of a triple layered flow that one furnish is used for the middle layerand the other one for the two outer layers. Both approach flow systems have to be dimensioned

Figure 7. Water Sankey diagram of supercalendered (SC) paper machine at design production.

Changes in the amount of stock circulating in the short circulation, i.e., by changes in the wireretention, have an effect on the stock concentration in the headbox. This includes slow variationsin retention, which might be initiated by changes in the amount, size distribution, or charge of thefillers and fines fraction, changes in the amount of dissolved and detrimental substances, or incationic charge demand, temperature, pH, etc.5.2.2.4 Headbox approach flow stabilityPipe bends, joints, and installations cause pipe flow disturbances, which appear as turbulenceand vortices causing a head loss4,5. Especially immediately before the headbox, secondary flowsor vortices have to be avoided as far as possible to ensure the most even flow conditions. Thenumber of bends in the pipe between the machine screen(s) and headbox should be as small aspossible, also for lessening the costs. Figure 3 shows one possible solution. The last bend beforethe headbox has a radius of at least three times the pipe diameter. If that is not possible for somereason, the pipe should be carried out as a so-called "hydraulic knee," where the pipe diameter isreduced over the bend. This solution is usually more expensive; however, in any case, cavitationand flow separation must be avoided. The bend is followed by a straight section of about fivetimes the pipe diameter right before the tapered inlet header of the headbox (if an attenuation unitor other special headbox approach is not used)6. No control valves or other measuring devicesare installed in the headbox feed pipe. Properties of the headbox feed stock are measured afterthe headbox at the recirculation line. See also the section about stock transport.

5.2.3 Stock and water balanceDetermining the stock and water balance of the paper mill is the starting point for any newprocess design as well as for an existing system analysis. The balances can vary substantially fordifferent situations, e.g., production of the lowest and the highest basis weight or in the case ofweb breaks. An illustrative manner of representing balances is in the form of a Sankey-diagramas shown in Figs. 6 and 7 for a supercalendered paper line in steady operation. Figure 6 showsthe flow of solids relative to the amount of paper at the reel, and Fig. 7 shows the specific waterflow for the same situation. Note that, for this grade, there is a large amount of solids in the shortcirculation in this case, at a retention of 54%. For clarity reasons, the figure shows the cleanerand screening system as one block each and the PM shower water system is very simplified.Collected waters are of different quality, and they are not fed all together into the white water(WW) tank, as shown in Figs. 6 and 7. Note also that the PM comprises a dilution water headbox.Dilution at the second and third cleaner stages is done with wire water, which is shown by thesmaller of the two parallel streams between the blocks "wire pit and fan pump" and "cleanersystem" in Figs. 6 and 7. The paper mill surplus water is usually fed upstream, like in this case,into the mechanical pulp production unit.

5.2.4 Multi-ply and multilayer systemsThe number of separate stock approach flow systems increases according to the number ofdifferent stock components used either in multi-ply or multilayer paper and board production.Figures 8 through 10 show the block diagrams for a printing paper-producing machine, amultilayer-producing PM like tissue or linerboard, and a multi-ply board or linerboard machine.

The multilayered sheet is produced from one multichannel headbox and one former; thus,there is only one wire pit (see Fig. 9). The multichannel headbox combines two or three stockflows into one jet leaving the headbox lip. This flow consists typically of two different furnishes,which means in the case of a triple layered flow that one furnish is used for the middle layerand the other one for the two outer layers. Both approach flow systems have to be dimensioned



according to the desired sheet structures and grades. The requirements on availability andtrouble-free operation of the stock approach flow systems are as high, if not even higher, as insingle systems because the failure of one approach flow system means that the entire paperproduction fails.

Figure 8. Printing paper machine, block diagram.

On a multi-ply board machine, either on a multifourdrinier machine or on a multiple formermachine, the wire waters are collected separately and the water circuits are usually separated(see Fig. 10). This is important, if the furnishes of the plies are of very different kinds, e.g., awhite top on a brown or gray mid or base layer. Investment costs are higher, due to the higherdegree of complexity of the entire stock approach flow system including separate blend andmachine chests for each ply or component. The larger amount of equipment also requires morefloor space, electric power, and instrumentation and control loops. The benefits of multi-plyforming are7:

- Production can be possibly increased,- Raw material costs can be optimized by using a cheaper furnish and still getting the same

strength or optical properties,- Better quality white-lined grades can be produced with lower basis weight of white pulp

layer.

Figure 9. Multilayer headbox machine with two stock components, block diagram.

Figure 10. Two-ply machine, block diagram.

5.2.5 System cleanlinessCleanliness refers to freedom from dirt and contaminants in the process and in the product.Cleanliness refers also to absence or to a low level in dissolved and colloidal materialcontamination, which can cause scale or dirt formed by precipitation, coagulation or biologicalactivity appearing as slime. Slime occurs as lumps or films causing product defects such asholes, specs, and smell, as well as production problems by plugging and scaling. The latter leadsto micro-biologically induced corrosion and possibly to decreased heat transfer. Micro-organicactivity is unavoidable due to the large content of nutrients in paper mill waters and the usuallyfavorable temperature. Hence, control of biological activity is needed in order to protect theproduction and the product from disturbing slime.

System cleanliness should be a concern everywhere in paper production. Cleanliness issupported by correct process design. On the one hand, standing water or low flow speed, deadends, edges, corners, rough surfaces, and low-quality materials have to be avoided in piping andmachinery where material can accumulate and slime or scaling can build up. Note, for example,that it is the first and not the last cleaner bank seen in feed flow direction in Fig. 3, which can bedisconnected, thereby ensuring full flow without dead ends in the distribution feed pipe. Figure 11shows that a fine finish of the steel surface hampers the growth of microbes8. This is important inthe stock approach system and nearer the headbox6, especially at locations where the flowvelocity is not high, like in the deaeration tank. On the other hand, cleaning of equipment andpipes has to be possible. This includes well-positioned wash fluid connectors and drainagevalves to empty pipes during shutdowns. The latter is essential for all thick stock pipes, whichshould be inclined and equipped with a drain at the lowest and a vent at the highest point. Airpockets are a prominent place for slime to build up, which then releases as lumps from time to

according to the desired sheet structures and grades. The requirements on availability andtrouble-free operation of the stock approach flow systems are as high, if not even higher, as insingle systems because the failure of one approach flow system means that the entire paperproduction fails.

Figure 8. Printing paper machine, block diagram.

On a multi-ply board machine, either on a multifourdrinier machine or on a multiple formermachine, the wire waters are collected separately and the water circuits are usually separated(see Fig. 10). This is important, if the furnishes of the plies are of very different kinds, e.g., awhite top on a brown or gray mid or base layer. Investment costs are higher, due to the higherdegree of complexity of the entire stock approach flow system including separate blend andmachine chests for each ply or component. The larger amount of equipment also requires morefloor space, electric power, and instrumentation and control loops. The benefits of multi-plyforming are7:

- Production can be possibly increased,- Raw material costs can be optimized by using a cheaper furnish and still getting the same

strength or optical properties,- Better quality white-lined grades can be produced with lower basis weight of white pulp

layer.

Figure 9. Multilayer headbox machine with two stock components, block diagram.

Figure 10. Two-ply machine, block diagram.

5.2.5 System cleanlinessCleanliness refers to freedom from dirt and contaminants in the process and in the product.Cleanliness refers also to absence or to a low level in dissolved and colloidal materialcontamination, which can cause scale or dirt formed by precipitation, coagulation or biologicalactivity appearing as slime. Slime occurs as lumps or films causing product defects such asholes, specs, and smell, as well as production problems by plugging and scaling. The latter leadsto micro-biologically induced corrosion and possibly to decreased heat transfer. Micro-organicactivity is unavoidable due to the large content of nutrients in paper mill waters and the usuallyfavorable temperature. Hence, control of biological activity is needed in order to protect theproduction and the product from disturbing slime.

System cleanliness should be a concern everywhere in paper production. Cleanliness issupported by correct process design. On the one hand, standing water or low flow speed, deadends, edges, corners, rough surfaces, and low-quality materials have to be avoided in piping andmachinery where material can accumulate and slime or scaling can build up. Note, for example,that it is the first and not the last cleaner bank seen in feed flow direction in Fig. 3, which can bedisconnected, thereby ensuring full flow without dead ends in the distribution feed pipe. Figure 11shows that a fine finish of the steel surface hampers the growth of microbes8. This is important inthe stock approach system and nearer the headbox6, especially at locations where the flowvelocity is not high, like in the deaeration tank. On the other hand, cleaning of equipment andpipes has to be possible. This includes well-positioned wash fluid connectors and drainagevalves to empty pipes during shutdowns. The latter is essential for all thick stock pipes, whichshould be inclined and equipped with a drain at the lowest and a vent at the highest point. Airpockets are a prominent place for slime to build up, which then releases as lumps from time to



time. Avoiding the buildup of air pockets is important when positioning valves and selecting theappropriate type of valve. At open surfaces, splashing should be avoided because air isentrained and material builds up on surfaces above the waterline. This then accumulates, dries,and eventually re-enters the process as detrimental chunks.

System washings have to be performed during planned maintenance shutdowns, which caninclude pressure rinse, high-temperature treatment, and washing with chemicals according toneeds9. Timer-controlled washing systems and showers can be installed on equipment andtanks, which are prone to build up dirt or chunks of dried stock. Finally, keeping floors andmachinery clean improves the mill operators' safety in general.

Figure 11. Microbe population on metal surfaces with different degree of finishing8.

5.3 Stock flow operations5.3.1 Stock blendingThe functional paper properties are determined in great part by the properties of the stockcomponents used. Type, quality, and quantity of the different components are determined by thespecific recipe for each grade. Therefore, the stock is a blend of several components in order toreach the desired paper properties under the most economic circumstances.

Generally speaking, stock blending can take place continuously or in a batch system. Inmodern papermaking, batch blending is used only for specialty papers produced on machineswith small production rates or even in discontinuous operation, applying very special furnishcomponents, dyes, or chemicals. Figure 12 shows a typical example for a continuous system. Allfiber components are diluted to the same pre-set concentration for blending. Each pulp typicallyhas a separate pulp chest, the proportioning chest, to ensure a constant supply at the dosagepoint. In an integrated mill, pulp is usually picked up at a medium-consistency storage tower bydilution with water from the main PM dilution header. The concentration in the pulp chest isusually adjusted to 0.2%0.3% points higher than in the blend chest. The stock is then diluted tothe blending concentration and pumped to blending via refiners or directly. The components areproportioned to the blend chest by flow metering and flow ratio controllers. The setpoints for thecontrollers are given to the process control system according to the current recipe. The levelcontroller regulates the total amount of stock entering the blend chest.

Occasionally, the blend chest is also called a "mixing chest." Despite the name, the functionof this chest is not only to create complete motion of the stock, which is referred to as "mixing,"but also to gain complete stock uniformity, referred to as "blending"10.

There are three or more components mixed in the blend chest:- Primary stock component(s), flow ratio controlled and consistency corrected- Broke, flow ratio controlled and consistency corrected- Recovered fiber from the saveall.

Possible consistency differences between the stock component flows can be corrected bycalculating the mass flow in the process control system. The components are typically fed via acommon header pipe to the blend chest. The header pipe at the side of the blend chest is also apossible dosage point for functional chemicals, e.g., dyes. The sweetener stock pump on theother side of this header pumps sweetener stock to the saveall disc filter. The amount of requiredsweetener can be large, cf. Fig. 6. The arrangement of the pipes determines which furnishcomponent is used predominantly as sweetener (see Fig. 12). The concentration in the blendchest is similar to the pre-set consistency of the blending streams. Under all possible productionconditions, the residence time of stock in the blend chest has to be longer than the blending time,

time. Avoiding the buildup of air pockets is important when positioning valves and selecting theappropriate type of valve. At open surfaces, splashing should be avoided because air isentrained and material builds up on surfaces above the waterline. This then accumulates, dries,and eventually re-enters the process as detrimental chunks.

System washings have to be performed during planned maintenance shutdowns, which caninclude pressure rinse, high-temperature treatment, and washing with chemicals according toneeds9. Timer-controlled washing systems and showers can be installed on equipment andtanks, which are prone to build up dirt or chunks of dried stock. Finally, keeping floors andmachinery clean improves the mill operators' safety in general.

Figure 11. Microbe population on metal surfaces with different degree of finishing8.

5.3 Stock flow operations5.3.1 Stock blendingThe functional paper properties are determined in great part by the properties of the stockcomponents used. Type, quality, and quantity of the different components are determined by thespecific recipe for each grade. Therefore, the stock is a blend of several components in order toreach the desired paper properties under the most economic circumstances.

Generally speaking, stock blending can take place continuously or in a batch system. Inmodern papermaking, batch blending is used only for specialty papers produced on machineswith small production rates or even in discontinuous operation, applying very special furnishcomponents, dyes, or chemicals. Figure 12 shows a typical example for a continuous system. Allfiber components are diluted to the same pre-set concentration for blending. Each pulp typicallyhas a separate pulp chest, the proportioning chest, to ensure a constant supply at the dosagepoint. In an integrated mill, pulp is usually picked up at a medium-consistency storage tower bydilution with water from the main PM dilution header. The concentration in the pulp chest isusually adjusted to 0.2%0.3% points higher than in the blend chest. The stock is then diluted tothe blending concentration and pumped to blending via refiners or directly. The components areproportioned to the blend chest by flow metering and flow ratio controllers. The setpoints for thecontrollers are given to the process control system according to the current recipe. The levelcontroller regulates the total amount of stock entering the blend chest.

Occasionally, the blend chest is also called a "mixing chest." Despite the name, the functionof this chest is not only to create complete motion of the stock, which is referred to as "mixing,"but also to gain complete stock uniformity, referred to as "blending"10.

There are three or more components mixed in the blend chest:- Primary stock component(s), flow ratio controlled and consistency corrected- Broke, flow ratio controlled and consistency corrected- Recovered fiber from the saveall.

Possible consistency differences between the stock component flows can be corrected bycalculating the mass flow in the process control system. The components are typically fed via acommon header pipe to the blend chest. The header pipe at the side of the blend chest is also apossible dosage point for functional chemicals, e.g., dyes. The sweetener stock pump on theother side of this header pumps sweetener stock to the saveall disc filter. The amount of requiredsweetener can be large, cf. Fig. 6. The arrangement of the pipes determines which furnishcomponent is used predominantly as sweetener (see Fig. 12). The concentration in the blendchest is similar to the pre-set consistency of the blending streams. Under all possible productionconditions, the residence time of stock in the blend chest has to be longer than the blending time,



i.e., the time until a reasonably homogenous mixture is reached. Therefore, agitation has to besufficient.

The blended pulp is pumped at a constant rate to the machine chest. The stock is diluted bya small concentration decrement, typically about 0.2%0.3%. All connections to the blend andmachine chest are designed in such a way that the best possible mixing occurs and entrainmentof air is low, e.g., by avoiding splashing and large vortices in the vicinity of the open surface. Insome installations, additional equipment like refiner or thick-stock screens can be found at theposition between the blend chest and the machine chest. Constant consistency and steadyhydraulic load at this position ease the operation of such equipment. On the other hand, it has tobe considered that any extra equipment at this location requires extra maintenance or mightotherwise cause extra downtime, which decreases the total PM efficiency. Uneven operation,e.g., by wear of refiner fittings or varying reject flow from the screen, can cause variations withadverse effects on the performance of the stock approach flow system.

Figure 12 shows also how a sampling site can be integrated into the blending system.Constant flow and the possibility to flush and to clean are important in order to collectrepresentative samples of the pulps for laboratory analysis. In addition, or instead of a station formanual sampling, automated or robotized pulp analysis equipment can be also installed.

Figure 12. Stock blending and machine chest including sampling station, an example.

5.3.2 Stock dosageThe basis weight of the sheet is controlled by the amount of thick stock from the machine chest tothe PM. It is affected by the amount of filler added to the short circulation. For information onbasis weight control, refer to Volume 14: Process Control of this book series. Stock dosage ormetering is commonly carried out in either of the following two alternative ways:

- By a thick stock valve right before dilution at the wire pit- By speed control of the thick stock feed pump.

In either case, the thick stock concentration is kept constant. Advantages of thespeed-controlled pump are a lower energy consumption, higher feed flow accuracy, and simplerpiping layout. The benefits are worth the effort, especially for machines producing a range ofgrades with large variation in the basis weight. A stuff box is not recommended in modern papermachines in either case because it is a source of slime problems.

If stock is metered by valve control, the basis weight valve is located directly before stockdilution at the wire pit. The pipe at the location of the valve should point upward in order to avoidthe accumulation of air on the downstream side of the valve. Valve control can be also carried outwith two valves in parallel, i.e., with one valve for coarse metering and the other one for finecontrol.5.3.3 Stock dilution5.3.3.1 PrincipleIn mill practice, reducing the stock consistency means the mixing of a high-consistency streamwith a low-consistency stream. Hence, consistency variations in the blended stream can becaused by variations in the flow and in the consistency of either of the streams to be blended.Consistency variations in the lean stream are usually not significant to the diluted stream, exceptif the rate of dilution is high. The flows and the size of the mixing volume or the mixing zonedetermine how much variation can be leveled out according to the amplitude and wavelength ofvariation. For example, the machine chest is dimensioned according to this principle.

5.3.3.2 Mixing

i.e., the time until a reasonably homogenous mixture is reached. Therefore, agitation has to besufficient.

The blended pulp is pumped at a constant rate to the machine chest. The stock is diluted bya small concentration decrement, typically about 0.2%0.3%. All connections to the blend andmachine chest are designed in such a way that the best possible mixing occurs and entrainmentof air is low, e.g., by avoiding splashing and large vortices in the vicinity of the open surface. Insome installations, additional equipment like refiner or thick-stock screens can be found at theposition between the blend chest and the machine chest. Constant consistency and steadyhydraulic load at this position ease the operation of such equipment. On the other hand, it has tobe considered that any extra equipment at this location requires extra maintenance or mightotherwise cause extra downtime, which decreases the total PM efficiency. Uneven operation,e.g., by wear of refiner fittings or varying reject flow from the screen, can cause variations withadverse effects on the performance of the stock approach flow system.

Figure 12 shows also how a sampling site can be integrated into the blending system.Constant flow and the possibility to flush and to clean are important in order to collectrepresentative samples of the pulps for laboratory analysis. In addition, or instead of a station formanual sampling, automated or robotized pulp analysis equipment can be also installed.

Figure 12. Stock blending and machine chest including sampling station, an example.

5.3.2 Stock dosageThe basis weight of the sheet is controlled by the amount of thick stock from the machine chest tothe PM. It is affected by the amount of filler added to the short circulation. For information onbasis weight control, refer to Volume 14: Process Control of this book series. Stock dosage ormetering is commonly carried out in either of the following two alternative ways:

- By a thick stock valve right before dilution at the wire pit- By speed control of the thick stock feed pump.

In either case, the thick stock concentration is kept constant. Advantages of thespeed-controlled pump are a lower energy consumption, higher feed flow accuracy, and simplerpiping layout. The benefits are worth the effort, especially for machines producing a range ofgrades with large variation in the basis weight. A stuff box is not recommended in modern papermachines in either case because it is a source of slime problems.

If stock is metered by valve control, the basis weight valve is located directly before stockdilution at the wire pit. The pipe at the location of the valve should point upward in order to avoidthe accumulation of air on the downstream side of the valve. Valve control can be also carried outwith two valves in parallel, i.e., with one valve for coarse metering and the other one for finecontrol.5.3.3 Stock dilution5.3.3.1 PrincipleIn mill practice, reducing the stock consistency means the mixing of a high-consistency streamwith a low-consistency stream. Hence, consistency variations in the blended stream can becaused by variations in the flow and in the consistency of either of the streams to be blended.Consistency variations in the lean stream are usually not significant to the diluted stream, exceptif the rate of dilution is high. The flows and the size of the mixing volume or the mixing zonedetermine how much variation can be leveled out according to the amplitude and wavelength ofvariation. For example, the machine chest is dimensioned according to this principle.

5.3.3.2 Mixing



Mixing is achieved by turbulence created by moving parts, surge on static elements, orturbulence created when feeding streams together. In coaxial pipe arrangements, like in wire pitstock dilution, the thick stock is fed into the dilution water and not vice versa, in order to gain thebest mixing and a stable flow. The streams are mixed by secondary flows, which are created bythe speed difference of the streams to be mixed. In wire pit stock dilution, the turbulence createdin the fan pump provides good mixing.

Consistency variations can also be filtered by dividing the flow into branches, in which thestock is retained for varying lengths of time before the separate flows are recombined. Figure 13shows schematically an arrangement of the multiple flow-lag principle through a dividedmanifold11. The stock flowing through the left-hand cleaner of the parallel cleaners has thelongest retention time. The principle applies in the same manner for pressure screens, deaerationvessel feed pipes, other piping, and the entire PM water system.

Figure 13. Multiple flow-lag principle.

5.3.3.3 Machine stock dilutionIn a typical stock approach system, the thick stock from the machine chest is diluted at thebottom of the wire pit (see Fig. 14). The diluted stock consistency depends on the retention, i.e.,the wire water consistency, and the amount of thick stock dosed, which again is adjusted to meetthe desired basis weight of the product. The actual consistency, which is the primary cleaner feedconsistency, can differ according to the produced grade, its basis weight, the current retention,filler content, etc. The thick stock consistency is kept constant. No consistency control occursafter the machine chest.

The geometry of the bottom as well as of the outlet of the wire pit is very important to ensurestable hydraulic conditions and a good mixing of the different components. A fan pump, which isfeeding the primary cleaner stage in most systems, is directly connected to the asymmetricallytapered mixing zone. Additives like dyes and possibly filler are dosed at the topside of thefan-pump suction piece. The coaxial flow of the main components is steady, if the flow velocitydifference between the components is large enough under all possible operation conditions:

V1 > V2 > V3 > V4where V1V4 are the flow speeds as shown in Table 2 and in Fig. 14. The outer concentric feedpipe is the end of the standpipe collecting circulation flows other than the wire water, possibly:

- Second- and third-stage cleaner accept- Second- and third-stage machine screen accept- Headbox recirculation, especially if there is no deaeration tank- Deaeration overflow- Overflow and circulation from the headbox dilution system.

Figure 14. PM wire pit with single stock dilution.

Feeding the thick stock and other circulated stock top-down at the fan-pump suction side,thus, at the position which is today exclusively used for additives (see Fig. 14), is an outdatedsolution. The disadvantages of this arrangement are, on the one hand, poor mixing due to thetype of arrangement itself and to low velocity differences. On the other hand, feed pipes that arearranged side-by-side can cause hydraulic interaction between the flows12.

Table 2. Common wire pit flow velocities as shown in Fig. 14.

Mixing is achieved by turbulence created by moving parts, surge on static elements, orturbulence created when feeding streams together. In coaxial pipe arrangements, like in wire pitstock dilution, the thick stock is fed into the dilution water and not vice versa, in order to gain thebest mixing and a stable flow. The streams are mixed by secondary flows, which are created bythe speed difference of the streams to be mixed. In wire pit stock dilution, the turbulence createdin the fan pump provides good mixing.

Consistency variations can also be filtered by dividing the flow into branches, in which thestock is retained for varying lengths of time before the separate flows are recombined. Figure 13shows schematically an arrangement of the multiple flow-lag principle through a dividedmanifold11. The stock flowing through the left-hand cleaner of the parallel cleaners has thelongest retention time. The principle applies in the same manner for pressure screens, deaerationvessel feed pipes, other piping, and the entire PM water system.

Figure 13. Multiple flow-lag principle.

5.3.3.3 Machine stock dilutionIn a typical stock approach system, the thick stock from the machine chest is diluted at thebottom of the wire pit (see Fig. 14). The diluted stock consistency depends on the retention, i.e.,the wire water consistency, and the amount of thick stock dosed, which again is adjusted to meetthe desired basis weight of the product. The actual consistency, which is the primary cleaner feedconsistency, can differ according to the produced grade, its basis weight, the current retention,filler content, etc. The thick stock consistency is kept constant. No consistency control occursafter the machine chest.

The geometry of the bottom as well as of the outlet of the wire pit is very important to ensurestable hydraulic conditions and a good mixing of the different components. A fan pump, which isfeeding the primary cleaner stage in most systems, is directly connected to the asymmetricallytapered mixing zone. Additives like dyes and possibly filler are dosed at the topside of thefan-pump suction piece. The coaxial flow of the main components is steady, if the flow velocitydifference between the components is large enough under all possible operation conditions:

V1 > V2 > V3 > V4where V1V4 are the flow speeds as shown in Table 2 and in Fig. 14. The outer concentric feedpipe is the end of the standpipe collecting circulation flows other than the wire water, possibly:

- Second- and third-stage cleaner accept- Second- and third-stage machine screen accept- Headbox recirculation, especially if there is no deaeration tank- Deaeration overflow- Overflow and circulation from the headbox dilution system.

Figure 14. PM wire pit with single stock dilution.

Feeding the thick stock and other circulated stock top-down at the fan-pump suction side,thus, at the position which is today exclusively used for additives (see Fig. 14), is an outdatedsolution. The disadvantages of this arrangement are, on the one hand, poor mixing due to thetype of arrangement itself and to low velocity differences. On the other hand, feed pipes that arearranged side-by-side can cause hydraulic interaction between the flows12.

Table 2. Common wire pit flow velocities as shown in Fig. 14.



Flow velocity [m/s]Thick stock V1 2.0 0.5Circulation V2 1.5 0.5Wire water V3 1.0 0.5Wire water V4 0.080.15

If a low headbox consistency is required, a two-stage dilution system might be needed. In thefirst stage, the stock is diluted to the cleaner feed consistency and, in the second stage, thecleaner accept is diluted further to reach the desired headbox consistency after screening.Hence, cleaner feed accept consistency and screen feed consistency are de-coupled. As anadvantage, the cleaner plant can be operated constantly at the optimal consistency, while theheadbox consistency may change according to the grade produced. Two-stage systems areusually considered for multiple grade producing machines with stock cleaning, if the headboxconsistency is about 0.7% or lower. On the contrary, tissue paper machines without cleaners andlittle product variations often have one-stage dilution, even though the consistency can be as lowas 0.15%. Similar to a multichannel headbox system with two approach flows, the wire pit alsohas for a traditional two-stage dilution two separate dilution zones, which are usually locatedopposite to each other. Another design consists of connected parallel wire water tanks each witha dilution zone. Such a system of connected wire water tanks has been applied in, e.g., multiplelayer production with two-stage dilution. However, the two-stage dilution system operates onlythen stable, if enough wire water is consumed at the second dilution stage.5.3.3.4 Headbox dilution systemThe aim of dilution at the headbox is to control the basis weight cross-profile at the PM (seeChapter 6). The amount of dilution water ranges from a few percent of the headbox lip flow up toover 20%. Basis weight control is more efficient the lower the solids content is of the dilutionwater13. To ensure efficient control, a certain dilution flow is needed at all dilution positions all thetime. According to the design and recommendations by the headbox manufacturer, the amount ofrequired dilution water can be quite high, even if there is a high one-pass retention like on boardmachines. In order to avoid an increase in hydraulic load on the saveall disc filter, the headboxdilution water is usually taken directly from the wire pit. For products with characteristically lowretention, the required water for dilution has to be increased accordingly. The approach flowsystem for the headbox dilution water consists of similar elements as the system for the dilutedstock main flow:

- A speed-controlled fan pump- A fine screen- Deaeration equipment similar to the stock system- Overflow from the headbox dilution header, usually similar to the stock flow.

The dilution water is deaerated in a separate unit or in a separate compartment, which isintegrated into the stock deaeration tank. The same design criteria are applied as for the stockapproach flow system and its equipment concerning low pulsation and the proper design ofsurfaces and materials.5.3.3.5 Medium- and high-consistency stock dilutionIn integrated mills, the prepared pulp is often collected from medium-consistency (MC) storagetowers, with a storage consistency of typically 8%12%. Dilution to a pumpable suspension takesplace in the bottom part of the storage tower by injecting dilution water, which is filtrate or whitewater taken from the main header of the PM water system. Figure 15 shows amedium-consistency storage tower. The dilution water is fed to the suction zone of the agitator or

Flow velocity [m/s]Thick stock V1 2.0 0.5Circulation V2 1.5 0.5Wire water V3 1.0 0.5Wire water V4 0.080.15

If a low headbox consistency is required, a two-stage dilution system might be needed. In thefirst stage, the stock is diluted to the cleaner feed consistency and, in the second stage, thecleaner accept is diluted further to reach the desired headbox consistency after screening.Hence, cleaner feed accept consistency and screen feed consistency are de-coupled. As anadvantage, the cleaner plant can be operated constantly at the optimal consistency, while theheadbox consistency may change according to the grade produced. Two-stage systems areusually considered for multiple grade producing machines with stock cleaning, if the headboxconsistency is about 0.7% or lower. On the contrary, tissue paper machines without cleaners andlittle product variations often have one-stage dilution, even though the consistency can be as lowas 0.15%. Similar to a multichannel headbox system with two approach flows, the wire pit alsohas for a traditional two-stage dilution two separate dilution zones, which are usually locatedopposite to each other. Another design consists of connected parallel wire water tanks each witha dilution zone. Such a system of connected wire water tanks has been applied in, e.g., multiplelayer production with two-stage dilution. However, the two-stage dilution system operates onlythen stable, if enough wire water is consumed at the second dilution stage.5.3.3.4 Headbox dilution systemThe aim of dilution at the headbox is to control the basis weight cross-profile at the PM (seeChapter 6). The amount of dilution water ranges from a few percent of the headbox lip flow up toover 20%. Basis weight control is more efficient the lower the solids content is of the dilutionwater13. To ensure efficient control, a certain dilution flow is needed at all dilution positions all thetime. According to the design and recommendations by the headbox manufacturer, the amount ofrequired dilution water can be quite high, even if there is a high one-pass retention like on boardmachines. In order to avoid an increase in hydraulic load on the saveall disc filter, the headboxdilution water is usually taken directly from the wire pit. For products with characteristically lowretention, the required water for dilution has to be increased accordingly. The approach flowsystem for the headbox dilution water consists of similar elements as the system for the dilutedstock main flow:

- A speed-controlled fan pump- A fine screen- Deaeration equipment similar to the stock system- Overflow from the headbox dilution header, usually similar to the stock flow.

The dilution water is deaerated in a separate unit or in a separate compartment, which isintegrated into the stock deaeration tank. The same design criteria are applied as for the stockapproach flow system and its equipment concerning low pulsation and the proper design ofsurfaces and materials.5.3.3.5 Medium- and high-consistency stock dilutionIn integrated mills, the prepared pulp is often collected from medium-consistency (MC) storagetowers, with a storage consistency of typically 8%12%. Dilution to a pumpable suspension takesplace in the bottom part of the storage tower by injecting dilution water, which is filtrate or whitewater taken from the main header of the PM water system. Figure 15 shows amedium-consistency storage tower. The dilution water is fed to the suction zone of the agitator or



injected along the propeller shaft directly into the zone of highest turbulence at the agitator14. Itsamount is usually controlled by the diluted stock flow. The fine adjustment of the diluted stockconsistency is by injection of dilution water at the suction side of the pump according to theconsistency measurement, as shown in Fig. 15.

For long distance transport of pulp in high consistency of 20%50%, either lorries or belt/tubeconveyors are used. High-consistency pulp is transported over short distances by screwconveyors. Stock from lorries is usually slushed in pulpers, while the use of a dilution screw ispossible for diluting the pulp, which is continuously fed by the conveyor. The pulp is diluted in thescrew to storage consistency and drops into a storage tower from where pulp proceeds asdescribed above.

The higher the transfer consistency is from integrated pulping to the paper machine system,the better is the separation of the water circuits of the systems. Thus, the less detrimentalsubstances are carried over into PM water, when the white water exchange is made at higheststock consistency.

Figure 15. Medium-consistency storage tower and stock dilution.

5.3.4 Cleaning and screeningChapter 9 of Volume 5: Mechanical Pulping of this book series describes the principles ofcleaning and screening and related equipment. Depending on the stock components, pulp hasalready been subjected to various classification processes like cleaning and screening upstream.Hence, the amount of foreign material to be removed is small; thus, the reject stream should besmall. Therefore end-stage cleaners are of special design, and screens are often not dischargingreject continuously in an effort to reduce the loss of fibers. The main purpose of cleaning andscreening is to ensure clean stock by removing bundles, flakes, and occasional debris andstrings, which are partly created within the system; refer to the above section about systemcleanliness.

5.3.4.1 Hydrocyclone cleaningCentrifugal cleaners are used to remove dense debris of fiber size or smaller from the dilutedstock within the short circulation15,16. This debris can be sand, grit, shives, pitch, or other denseparticles. Practically all low basis weight and printing paper machines have a multistage cleanercascade system, which can be attached to a deaeration tank.

In the hydrocyclone, the suspension path involves a double vortex with the suspensionspiraling downward at the outside and upward at the inside. At the beginning of the conical part ofthe cyclone, the stream velocity undergoes a redistribution so that the tangential component ofvelocity increases with decreasing radius. The spiral velocity in the cyclone might reach a valueseveral times the inlet velocity. The separation of accepted and rejected particles depends on thevelocity profile and the location of the layer of zero-vertical velocity. The smaller the maindiameter of the cyclone is, the more efficient is the separation of debris but the smaller is thehydraulic capacity. At small diameter, the risk of plugging is higher. In the PM cleaner cascade,however, plugging is a minor problem due to dilution after every stage and due to oversizedparticle removal already performed during stock preparation by screening or high-densitycleaners.

No cutoff size or critical particle diameter exists for cyclone separation. Centrifugal and shearforces determine the separation or fractionation1719. The flow pattern in the cyclone is verycomplex, and the separation efficiency curve is unique for a given cleaner geometry. Figure 16shows the flow pattern schematically. Hence, the debris removal efficiency must be determinedexperimentally. The conical part of the cleaner can contain baffles and helical guides to modify or

injected along the propeller shaft directly into the zone of highest turbulence at the agitator14. Itsamount is usually controlled by the diluted stock flow. The fine adjustment of the diluted stockconsistency is by injection of dilution water at the suction side of the pump according to theconsistency measurement, as shown in Fig. 15.

For long distance transport of pulp in high consistency of 20%50%, either lorries or belt/tubeconveyors are used. High-consistency pulp is transported over short distances by screwconveyors. Stock from lorries is usually slushed in pulpers, while the use of a dilution screw ispossible for diluting the pulp, which is continuously fed by the conveyor. The pulp is diluted in thescrew to storage consistency and drops into a storage tower from where pulp proceeds asdescribed above.

The higher the transfer consistency is from integrated pulping to the paper machine system,the better is the separation of the water circuits of the systems. Thus, the less detrimentalsubstances are carried over into PM water, when the white water exchange is made at higheststock consistency.

Figure 15. Medium-consistency storage tower and stock dilution.

5.3.4 Cleaning and screeningChapter 9 of Volume 5: Mechanical Pulping of this book series describes the principles ofcleaning and screening and related equipment. Depending on the stock components, pulp hasalready been subjected to various classification processes like cleaning and screening upstream.Hence, the amount of foreign material to be removed is small; thus, the reject stream should besmall. Therefore end-stage cleaners are of special design, and screens are often not dischargingreject continuously in an effort to reduce the loss of fibers. The main purpose of cleaning andscreening is to ensure clean stock by removing bundles, flakes, and occasional debris andstrings, which are partly created within the system; refer to th

stock and water systems of the paper machine

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